FCC separation apparatus with improved stripping

ABSTRACT

In this invention a cyclonic separation apparatus discharges particulate solids and gaseous fluids into a separation vessel from a discharge opening of a central conduit and withdraws separated gaseous fluids from the separation vessel that contacts the catalyst in the separation vessel with redistributed gases from outside the separation vessel. The invention increases the effective utilization of available stripping medium in an FCC process.

CROSSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. Ser. No. 08/364,621, filed Dec.27, 1994, and now issued as U.S. Pat. No. 5,584,985.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to processes for the separation ofsolid catalyst particles from gases and the stripping of hydrocarbonsfrom catalyst. More specifically, this invention relates to theseparation of catalyst and gaseous materials from a mixture thereof in acyclonic disengaging vessel of an FCC process.

2. Description of the Prior Art

Cyclonic methods for the separation of solids from gases are well knownand commonly used. A particularly well known application of such methodsis in the hydrocarbon processing industry were particulate catalystscontact gaseous hydrocarbon reactants to effect chemical conversion ofthe gas stream components or physical changes in the particlesundergoing contact with the gas stream.

The FCC process presents a familiar example of a process that uses gasstreams to contact a finally divided stream of catalyst particles andeffects contact between the gas and the particles. The FCC processes, aswell as separation devices used therein are fully described in U.S. Pat.Nos. 4,701,307 and 4,792,437, the contents of which are herebyincorporated by reference.

The most common method of separating particulate solids from a gasstream uses a cyclonic separation. Cyclonic separators are well knownand operate by imparting a tangential velocity to a gases containingentrained solid particles that forces the heavier solids particlesoutwardly away from the lighter gases for upward withdrawal of gases anddownward collection of solids. Cyclonic separators usually compriserelatively small diameter cyclones having a tangential inlet on theoutside of a cylindrical vessel that forms the outer housing of thecyclone.

Cyclones for separating particulate material from gaseous materials arewell known to those skilled in the art of FCC processing. In theoperation of an FCC cyclone tangential entry of the gaseous materialsand catalyst creates a spiral flow path that establishes a vortexconfiguration in the cyclone so that the centripetal accelerationassociated with an outer vortex causes catalyst particles to migratetowards the outside of the barrel while the gaseous materials enter aninner vortex for eventual discharge through an upper outlet. The heaviercatalyst particles accumulate on the side wall of the cyclone barrel andeventually drop to the bottom of the cyclone and out via an outlet and adip leg conduit for recycle through the FCC arrangement. Cyclonearrangements and modifications thereto are generally disclosed in U.S.Pat. Nos. 4,670,410 and 2,535,140.

The FCC process is representative of many processes for which methodsare sought to quickly separate gaseous fluids and solids as they aredischarged from a conduit. In the FCC process one method of obtainingthis initial quick discharge is to directly connect a conduit containinga reactant fluid and catalyst directly to a traditional cycloneseparators. While improving separation, there are drawbacks to directlyconnecting a conduit discharging a mixture of solids and gaseous fluidsinto cyclone separators. Where the mixture discharged into the cyclonescontains a high loading of solids, direct discharge requires largecyclones. In addition, instability in the delivery of the mixture mayalso cause the cyclones to function poorly and to disrupt the processwhere pressure pulses cause an unacceptable carryover of solids with thehydrocarbon vapor separated by the cyclones. Such problems arefrequently encountered in processes such as fluidized catalyticcracking. Accordingly, less confined systems are often sought to effectan initial separation between a mixture of solid particles and gaseousfluids.

U.S. Pat. Nos. 4,397,738 and 4,482,451, the contents of which are herebyincorporated by reference, disclose an alternate arrangement forcyclonic separation that tangentially discharges a mixture of gases andsolid particles from a central conduit into a containment vessel. Thecontainment vessel has a relatively large diameter and generallyprovides a first separation of solids from gases. This type ofarrangement differs from ordinary cyclone arrangements by the dischargeof solids from the central conduit and the use of a relatively largediameter vessel as the containment vessel. In these arrangements theinitial stage of separation is typically followed by a second morecompete separation of solids from gases in a traditional cyclone vessel.

In addition to the separation of the solid catalyst from the gases,effective operation of the FCC process also requires the stripping ofhydrocarbons from the solid catalyst as it passes from the reactor to aregenerator. Stripping is usually accomplished with steam that displacesadsorbed hydrocarbons from the surface and within the pores of the solidcatalytic material. It is important to strip as much hydrocarbon aspossible from the surface of the catalyst to recover the maximum amountof product and minimize the combustion of hydrocarbons in theregenerator that can otherwise produce excessive temperatures in theregeneration zone.

U.S. Pat. No. 4,689,206 discloses a separation and stripping arrangementfor an FCC process that tangentially discharges a mixture of catalystand gases into a separation vessel and passes gases upwardly from alower stripping zone into a series of baffles for displacinghydrocarbons from the catalyst within the separation vessel. While thearrangement shown in U.S. Pat. No. 4,689,206 may effect some strippingof hydrocarbon gases from the catalyst in the separation vessel, thearrangement does not utilize all of the available gases for stripping ofthe hydrocarbons in the separation vessel and does not distribute thestripping gas that enters the separation vessel in a manner that insuresits effective use via good dispersion within the catalyst phase.

While it is beneficial to effect as much stripping and recover as manyhydrocarbons as possible from FCC catalyst, refiners have come underincreasing pressure to reduce the amount of traditional stripping mediumthat are used to effect stripping. The pressure stems from thedifficulty of disposing the sour water streams that are generated by thecontacting the catalyst with steam in typical stripping operations.Therefore, while more efficient process operations call for the use ofmore effective hydrocarbon stripping from FCC catalyst, the quantitiesof the preferred stripping mediums are being restricted.

BRIEF SUMMARY OF THE INVENTION

It has now been discovered that the stripping efficiency of a cyclonicseparation that centrally discharges particles into a separation chambermay be surprisingly improved by operating a reactor vessel in a specificmanner that channels all of the available stripping gases into theseparation vessel while simultaneously distributing the gases in amanner that increases the effectiveness of stripping in the separationchamber. In accordance with this discovery the gaseous fluids in thereactor vessel that surround the separation chamber are maintained at ahigher pressure within the reactor vessel than the pressure within theseparation chamber. The higher pressure creates a net gas flow from thevolume of the reactor vessel that surrounds the separation chamber intothe separation vessel. The effectiveness of the stripping is enhanced bydirecting some or all of this gas into a catalyst bed within theseparation chamber at a location above the bottom of the separationchamber across a plurality of flow restrictions. The flow restrictionsinsure that gases entering the separation chamber will have a uniformdistribution thatputs the gas to effective use as a stripping medium.

Accordingly, in one embodiment this invention is a process for thefluidized catalytic cracking of a hydrocarbon feedstock. The processpasses hydrocarbon feedstock and solid catalyst particles into a riserconversion zone comprising a conduit to produce a mixture of solidparticles and gaseous fluids. The mixture passes into a separationvessel through the conduit wherein the conduit occupies a centralportion of the separation vessel and the separation vessel is locatedwithin a reactor vessel. The conduit tangentially discharging themixture from a discharge opening into the separation vessel. Catalystparticles pass into a first catalyst bed located in a lower portion ofthe separation vessel and contact the catalyst particles with a firststripping gas in the first bed. Catalyst particles pass from the firstbed into a second bed located in the separation vessel below the firstcatalyst bed. Catalyst particles contact a second stripping gas and thesecond stripping gas passes into the first catalyst bed to supply aportion of the first stripping gas. The catalyst particles from thesecond bed pass to a stripping zone and contact a third stripping gas inthe stripping zone. The third stripping gas passes into the secondcatalyst bed to supply at least a portion of the second stripping gas. Apurge medium passes into an upper portion of the reactor vessel and atleast a portion of the purge gas passes through a plurality ofrestricted opening arranged circumferentially around the outside of theseparation vessel at the bottom of the first catalyst bed to supply aportion of the first stripping gas. Stripped catalyst particles arerecovered from the first stripping zone. An outlet withdraws collectedgaseous fluids including the first stripping gas and catalyst particlesfrom an upper portion of the separation vessel into an outlet andwithdraws gaseous fluids from the separation vessel.

In another embodiment this invention is an apparatus for separatingsolid particles from a stream comprising a mixture of gaseous fluids andsolid particles. The apparatus comprises a reactor vessel; a separationvessel located in the reaction vessel; and a mixture conduit extendinginto the separation vessel and defining a discharge opening locatedwithin the vessel. The discharge opening is tangentially oriented fordischarging the stream into the vessel and imparting a tangentialvelocity to the stream. A particle outlet defined by the separationvessel discharges particles from a lower portion of the vessel. Astripping vessel is located below the separation vessel. A gas recoveryconduit defines an outlet for withdrawing gaseous fluids from within theseparation vessel and a cyclone separator is in communication with thegas recovery conduit. A plurality of nozzles are located above thebottom of the separation vessel and extend circumferentially around theseparation vessel for communicating the separation vessel with thereactor vessel.

By maintaining the a bed of catalyst in the separation vessel andinjecting stripping fluid from the reactor vessel into the dense bed ofthe separation vessel at a location above the bottom of the separationvessel all available gases in the reactor vessel are used as strippingmedium. Such gases include the purge gas that enters the top of thereactor vessel to displace hydrocarbons that collect at the top of thevessel as well as cracked hydrocarbon gases from the dip legs of thecyclones. The cracked gases from the dip legs of the cyclones areparticularly effective as stripping gases since they have undergonecracking to the point of being essentially inert as a result of the longresidence time in the cyclone dip legs. Using all of the gases that arealready present in the reactor vessel as a stripping medium that passesthrough the separation vessel can reduce the total requirements forstripping steam that would otherwise be needed to achieve a desireddegree of stripping. Eliminating steam requirements is particularlybeneficial to refiners that are increasingly faced with treating costsassociated with the disposal of the sour water generated thereby.

In addition, the method and apparatus of this invention can furtherreduce steam requirement by utilizing the available stripping gas in amore effective manner that has been utilized in the past. Prior artarrangements for stripping catalyst in a separation vessel admit thestripping gas through the typically large bottom opening of theseparation vessel. The gas does not generally enter such an openinguniformly and tends to flow in primarily to one side or the other.Injecting the stripping gas from the reactor vessel into the dense bedof the separation vessel across a plurality of nozzles distributes thestripping gas in a manner that uniformly injects the stripping gas overthe circumference of the vessel. With this manner of distribution thegas is used effectively as a stripping medium.

Additional details and embodiments of the invention will become apparentfrom the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a sectional elevation of an FCC reactor vesselschematically showing a separation vessel arranged in accordance withthis invention.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus of this invention comprises a separation vessel into whicha mixture conduit that contains the mixture of solid particlestransported by a gaseous fluid discharges the particles and gaseousfluid mixture. The separation vessel is preferably a cylindrical vessel.The cylindrical vessel promotes the swirling action of the gaseousfluids and solids as they are discharged tangentially from a dischargeopening of the mixture conduit into the separation vessel. Theseparation vessel will preferably have an open interior below thedischarge opening that will still provide satisfactory operation in thepresence of some obstructions such as conduits or other equipment whichmay pass through the separation vessel.

The discharge opening and the conduit portion upstream of the dischargeopening are constructed to provide a tangential velocity to the exitingmixture of gaseous fluids and solids. The discharge opening may bedefined using vanes or baffles that will impart the necessary tangentialvelocity to the exiting gaseous fluids and solids. Preferably thedischarge outlet is constructed with conduits or arms that extendoutwardly from a central mixture conduit. Providing a section of curvedarm upstream of the discharge conduit will provide the necessarymomentum to the gaseous fluids and solids as they exit the dischargeopening to continue in a tangential direction through the separationvessel. The separation vessel has an arrangement that withdraws catalystparticles from the bottom of the vessel so that the heavier solidparticles disengage downwardly from the lighter gaseous fluids. A bed ofsolid particles is maintained at the bottom of the separation vesselthat extends into the separation vessel. The separated gases from theseparation vessel will contain additional amounts of entrained catalystthat are typically separated in cyclone separators. Preferred cycloneseparators will be of the type that having inlets that are directlyconnected to the outlet of the separation vessel. Additional details ofthis type of separation arrangement may be obtained from previouslyreferenced U.S. Pat. No. 4,482,451.

An essential feature of this invention is the location of a plurality ofrestricted openings arranged circumferentially around the outside of theseparation vessel. The outlets are located above the bottom outlet ofthe separation vessel and below the top of the dense catalyst phasemaintained within the separation vessel. To insure good distribution therestricted openings create a pressure drop of at least 0.25 psi. Therestricted openings are preferably in the form of nozzles that provideorifices to direct the gas flow into the dense catalyst phase of theseparation vessel. The nozzles will preferably have orifice openingdiameters of 1 inch or less and a spacing around the circumference ofthe separation vessel of less 12 inches and more preferably less than 6inches. To obtain a uniform pressure drop all of the restricted openingsare preferably located at the same elevation in the wall of theseparation vessel.

The gas flows into the reactor vessel that can enter the restrictedopenings of the separation vessel as stripping medium come from avariety of sources. The primary source is the purge medium that entersthe reactor vessel. In the absence of the purge, the volume of thereactor vessel that surrounds the separation chamber and a directconnected cyclones arrangement would remain relatively inactive duringthe reactor operation. The purge medium provides the necessary functionof sweeping the otherwise relatively inactive volume free ofhydrocarbons that would otherwise lead to coke formation in the vessel.Since this purge medium is usually steam it readily supplies a potentialstripping gas. Another stripping medium is available from the catalystoutlets of the cyclones. The recovered catalyst exiting the cyclonescontains additional amounts of entrained gases that enter the reactorvessel. As mentioned previously, these gases are rendered relativelyinert by a long residence time in the cyclone dip legs that cracks theheavy components to extinction.

The effective utilization of the stripping gas streams from the reactorvessel in the manner of this invention employs a particular pressurebalance between the separation vessel, the surrounding reactorenvironment, and the restricted openings. The pressure balance of thisinvention maintains a higher pressure in the reactor vessel than theseparation vessel. Maintaining the necessary pressure balance demandsthat a dense catalyst phase extend upward in the reactor above thebottom and into the separation vessel. For the purposes of thisinvention a dense catalyst phase is defined as a catalyst density of atleast 20 lb/ft³. The dense catalyst phase extends upward within thelower portion of the separation vessel to a height above the restrictedopenings. As hereinafter explained in the specific embodiment, theheight of the dense catalyst phase above the restricted openings islimited by the maximum differential pressure across the cyclones fromthe cyclone inlet to the dip pipe outlet. The maximum differentialacross the cyclones can be increased by increasing the length of thecyclone dip leg.

The restricted openings or nozzles are located above the bottom of theseparation vessel to maintain a head of dense catalyst between therestricted openings and the bottom of the separation vessel. This headof catalyst forces at least a portion of the gases from the reactor toflow into the separation vessel through the restricted openings insteadof the bottom separation vessel opening since, in accordance with thisinvention, the pressure in the reactor vessel always exceeds thepressure in the separation vessel at the restricted openings. Preferablythe head of catalyst in the separation vessel below the restrictedopenings will remain greater than the pressure drop across therestricted openings so that all of the gas from the reactor vessel willflow through the restricted openings and undergo redistribution beforestripping catalyst in the separation vessel.

The pressure balance requirements and operation of the process are morefully described in the following description of the preferredembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Looking then at the FIGURE, the schematic illustration depicts aseparation arrangement in a reactor vessel 10. A central conduit in theform of a reactor riser 12 extends upwardly from a lower portion of thereactor vessel 10 in a typical FCC arrangement. The central conduit orriser preferably has a vertical orientation within the reactor vessel 10and may extend upwardly from the bottom of the reactor vessel ordownwardly from the top of the reactor vessel. Riser 12 terminates in anupper portion of a separation vessel 11 with an curved conduit in theform of an arm 14. Arm 14 discharges a mixture of gases fluids and solidparticles comprising catalyst.

Tangential discharge of gases and catalyst from a discharge opening 16produces a swirling helical pattern about the interior of separationvessel 11 below the discharge opening 16. Centripetal accelerationassociated with the helical motion forces the heavier catalyst particlesto the outer portions of separation vessel 11. Catalyst from dischargeopenings 16 collects in the bottom of separation vessel 11 to form adense catalyst bed 17.

The gases, having a lower density than the solids, more easily changedirection and begin an upward spiral with the gases ultimately travelinginto a gas recovery conduit 18 having an inlet 20 that serves as the gasoutlet for separation vessel 11. In a preferred form of the invention(not depicted by the FIGURE) inlet 20 is located below the dischargeopening 16. The gases that enter gas recovery conduit 18 through inlet20 will usually contain a light loading of catalyst particles. Inlet 20recovers gases from the discharge conduit as well as stripping gaseswhich are hereinafter described. The loading of catalyst particles inthe gases entering conduit 18 are usually less than 1 lb/ft.³ andtypically less than 0.1 lb/ft³.

Gas recovery conduit 18 passes the separated gases into a cyclones 22that effect a further removal of particulate material from the gases inthe gas recovery conduit. Cyclones 22 operate as conventional directconnected cyclones in a conventional manner with the tangential entry ofthe gases creating a swirling action inside the cyclones to establishthe well known inner and outer vortexes that separate catalyst fromgases. A product stream, relatively free of catalyst particles, exitsthe reactor vessel 10 through outlets 24.

Catalyst recovered by cyclones 22 exits the bottom of the cyclonethrough dip-leg conduits 23 and passes through a lower portion of thereactor vessel 10 where it collects with catalyst that exits separationvessel 11 through an open bottom 19 to form a dense catalyst bed 28having an top surface 28' in the portion outside the separator vessel 11and a top surface 28" within separation vessel 11. Catalyst fromcatalyst bed 28 passes downwardly through a stripping vessel 30. Astripping fluid, typically steam enters a lower portion of strippingvessel 30 through a distributor 31. Countercurrent contact of thecatalyst with the stripping fluid through a series of stripping baffles32 displaces product gases from the catalyst as it continues downwardlythrough the stripping vessel. Fluidizing gas or additional strippingmedium may be added at the top of catalyst bed 28 by distributor 29.

Stripped catalyst from stripping vessel 30 passes through a conduit 15to a catalyst regenerator 34 that rejuvenates the catalyst by contactwith an oxygen-containing gas. High temperature contact of theoxygen-containing gas with the catalyst oxidizes coke deposits from thesurface of the catalyst. Following regeneration catalyst particles enterthe bottom of reactor riser 12 through a conduit 33 where a fluidizinggas from a conduit 35 pneumatically conveys the catalyst particlesupwardly through the riser. As the mixture of catalyst and conveying gascontinues up the riser, nozzles 36 inject feed into the catalyst, thecontact of which vaporizes the feed to provide additional gases thatexit through discharge opening 16 in the manner previously described.

The volume of the reactor outside cyclones 22 and separation vessel 11,referred to as outer volume 38, is kept under a positive pressure, P₂,relative to the pressure, P₃, inside the cyclones and the pressure P₁,in the separation vessel by the addition of a purge medium that entersthe top of the vessel through a nozzle 37. The purge medium typicallycomprises steam and is used to maintain a low hydrocarbon partialpressure in outer volume 38 to prevent the problem of coking aspreviously described.

This invention adds the restricted openings in the form of nozzles 40 sothat all of the purge medium entering nozzle 37 is effectively used as astripping or prestripping medium in an upper portion 41 of densecatalyst bed 17. The minimum positive pressure P2 is equal to thepressure, P_(RX), of the reactants at the outlets 16, the pressure dropassociated with the head of catalyst above the nozzles 40 and anyadditional pressure drop across nozzles 40. If the pressure drop acrossthe nozzles 40 is ignored the minimum positive pressure is equal to P1.The height of dense catalyst bed portion 41, indicated as X in theFIGURE, is essential to the operation of this invention since itprovides the location for full utilization of the available strippingmedium by the initial stripping of the majority of the catalyst as itenters the separation vessel. Height X will usually extend upward for atleast a foot. As discussed earlier the height X is limited by theavailable length of dip leg 23. As height X increases, the additionalcatalyst head raises the value of pressure P1 and the minimum pressurefor P2. Since pressure P3 equals the pressure PRX minus the cyclonepressure drop, pressure in the upper part of the cyclone remainsconstant relative to PRX. Therefore, raising pressure P2 at the bottomof dip leg 23 increases the level of dense catalyst within dip pipe 23.As a result the height X must be kept below a level that would causedense catalyst level 42 to enter the barrel portion 43 of cyclones 22.Thus in a preferred form of the invention, the pressure P2 is regulatedon the basis of the catalyst level in separation vessel 11.

The maximum value of pressure P2 is also limited relative to pressure P1by the distance that the lower portion 44 of bed 17 extends belownozzles 40. Once the pressure P2 exceeds pressure P1 by an amount equalto the head of catalyst over height Y, gas from outer volume 38 willflow under the bottom of the separation vessel and into its interiorthrough opening 19. Thus, the height Y serves as a limitation on thepressure drop through nozzles 40 which can never exceed the pressuredeveloped by the head of catalyst over height Y. Therefore, there is nolimitation on the amount of purge medium that can enter the processthrough nozzle 37 and any additional amounts of stripping or purge gasthat enter the regenerator vessel flow in to the separation vesselthrough bottom opening 19. In order to capture as much availablestripping medium as possible for redistribution and stripping inseparation vessel 11, height Y will provide a minimum distancecorresponding to the desired pressure drop across nozzles 40 toeliminate the flow of gas into bottom opening 19. As the pressure dropacross nozzles 40 decreases to the point of preventing gas flow from theouter volume 38 through the bottom opening 19, the top of bed 28 willlie somewhere between bed level 28' and the elevation of nozzles 40.Further decreases in flow of purge gas will bring the top level of bed28 close to nozzles 40. Preferably the height Y of catalyst ismaintained such that all of the gaseous materials in outer volume 38passes through nozzles 40 without gas flowing into separation vessel 11through opening 19. In most arrangements the distance Y will equal atleast 12 inches. Thus, in the preferred arrangement all of the strippinggas from bed 28 will flow into bed portion 44 and all of the strippinggas from bed portion 44 along with the gas from outer volume 38 willflow through bed portion 41 as a stripping medium.

What is claimed is:
 1. An apparatus for separating solid particles froma stream comprising a mixture of gaseous fluids and solid particles,said apparatus comprising:a reactor vessel; a separation vessel locatedin said reaction vessel; a mixture conduit extending into saidseparation vessel and defining a discharge opening located within saidvessel and tangentially oriented for discharging said stream into saidvessel and imparting a tangential velocity to said stream; a particleoutlet defined by said separation vessel for discharging particles froma lower portion of said vessel; a stripping vessel located below saidseparation vessel; a gas recovery conduit defining an outlet forwithdrawing gaseous fluids from said separation vessel; and, a pluralityof nozzles located above the bottom of said separation vessel andextending circumferentially around said separation vessel forcommunicating said separation vessel with said reactor vessel.
 2. Theapparatus of claim 1 wherein a cyclone separator is in communicationwith said gas recovery conduit.